I'm planning a sci fi story with things with portals, time machines, etc.
Would it be believable to say these gadgets are being developed by the government secretly (kind of like Men in Black). The story takes place in the future.
Depends how far in the future, if it's like 50 to a 100 years then it could be by the government. But if its like a thousand years in the future then it could have just been invented sometime before then.